Hybridoma technology has been utilized to produce 34 monoclonal antibodies which immunoprecipitate virion proteins of the transforming primate herpes-virus, Herpes virus saimiri. This battery of antibodies, which collectively recognize virtually all of the major structural antigens of the virus, offers a unique opportunity to analyze the replication and mode of transformation of this virus. Sensitive competition radiommunoassays which detect nanogram levels of the major structural proteins of two interesting retroviruses, caprine arthritis-encephalitis virus (CAEV) and equine infectious anemia virus (EIAV), have been developed. To date, it has been possible to measure the degree of immunological relatedness between CAEV and the prototype lentivirus, visna virus. Further characterization of immunological relationship between these viruses and other retroviruses is currently underway.